DOCS_7945 - Added clarifications to covered query explanation and example

source/core/query-optimization.txt

@@ -92,7 +92,9 @@ index use <regex-index-use>`.
 Covered Query
 -------------
 
-An index :ref:`covers <indexes-covered-queries>` a query when both of the
+A covered query is a query that can be satisfied entirely using an index
+and does not have to examine any documents. An index
+:ref:`covers <indexes-covered-queries>` a query when both of the
 following apply:
 
 - all the fields in the :ref:`query <read-operations-query-document>`
@@ -161,15 +163,16 @@ An index **cannot** cover a query if:
 
      { "user.login": 1 }
 
-  The ``{ "user.login": 1 }`` index does **not** cover the
-  following query:
+  The ``{ "user.login": 1 }`` index will **not** cover the
+  query below because this index is on a field in an embedded document:
 
   .. code-block:: none
 
      db.users.find( { "user.login": "tester" }, { "user.login": 1, _id: 0 } )
 
-  However, the query can use the ``{ "user.login": 1 }`` index to find
-  matching documents.
+  This query can still use the ``{ "user.login": 1 }`` index to find
+  matching documents but it will examine and fetch documents to satisfy the
+  query.
 
 .. _covered-index-sharded-collection: